1. Field of the Invention
The present invention relates to an output circuit of a charge mode sensor.
2. Description of the Related Art
Output circuits for charge mode sensors such as piezoelectric elements in related art include charge-voltage conversion circuits (charge amplifier circuits) that accumulate the charge of the sensors in capacitors to acquire output voltages from the capacitors and current-voltage conversion circuits that measure the amount of current passing through resistors as the amount of charge of the sensors to acquire output voltages from the current.
Japanese Unexamined Patent Application Publication No. 2007-51930 discloses a signal processing apparatus for a charge mode sensor. In the signal processing apparatus, abnormality of the charge mode sensor is detected in a charge-voltage conversion circuit (charge amplifier circuit).
Since the insulation resistance of a sensor cable and the input impedance of an operational amplifier are practically finite in the charge-voltage conversion circuit (charge amplifier circuit) in the related art, part of the charge generated in the charge mode sensor leaks through the insulation resistor of the sensor cable or the input terminal of the operational amplifier. As a result, measurement error (reduction in sensitivity and/or zero-point drift), particularly, the measurement error at low frequencies or in long-time measurement is increased.
It is not possible to avoid or reduce the problem of the measurement error with the signal processing apparatus using the charge-voltage conversion circuit (charge amplifier circuit) disclosed in Japanese Unexamined Patent Application Publication No. 2007-51930. In contrast, it is possible to reduce the measurement error (reduction in sensitivity) due to the insulation resistance and the input impedance with the current-voltage conversion circuit in the related art.
However, with the current-voltage conversion circuit in the related art, it is not possible to detect any fault (break or short circuit) occurring in the charge mode sensor or the sensor cable circuit if no charge is made. Accordingly, in order to confirm any fault before the measurement of the charge, it is necessary to additionally prepare a fault detection circuit that has no effect on the circuit.
In addition, use of the measured temperature of the charge mode sensor in temperature compensation of measurement data is not considered in the conversion circuits in both modes in the related art.